The invention has particular although not limited reference to systems in which a relatively high temperature, confined fluid flows over a heat transfer surface, fin material being brazed to the surface for an improved coefficient of heat transfer from the flowing fluid to and through the transfer surface.
In the use of such systems the fins should be soundly brazed to the heat transfer surface. This is important for heat transfer considerations, of course, but it also important to avoid problems brought about by fragmented fin material becoming entrained in the flowing fluid and having disruptive or damaging effects in the system downstream of the heat transfer surface.
A braze alloy defines with joined parts a braze joint. Ideally, the braze joint is dense and free of voids. In actual practice, however, voids do occur praticularly at locations of entrapped organic matter, as for example oil, grease, flux and like materials. In use, and under conditions of high temperature cycling, foreign materials in the braze joint vaporize, and, in expanding, form pressure pockets. These appear as visually distinguishable blisters on brazed fin portions. The fin material is weakened at blister locations and under the effects of vibration, fluid flow and pressure, can disintegrate and become entrained in the flowing fluid. Rising temperatures also produce expansion stresses, particularly when the fin and heat transfer surface are made of different material, and these stresses seek voids and result also in surface irregularities which may generally be termed blisters. Weakly brazed areas appear when inadequate clamping results in localized poor contact of brazed parts with one another during the brazing process. These areas, too, are subject to blistering and fragmenting in use.